Use of wireless networks is becoming widespread in business (commercial, industrial, institutional) and also consumer markets for the automatic control of various device arrangements. Examples are building automation systems, e.g. for lighting, heating & ventilation, safety, etc., comprising devices such as light ballasts, switches, dimmers or other control elements, daylight/occupancy sensors, actuators, meters etc. Use of wireless control makes the automation devices independent of the mains power as control medium, thus allowing freedom of device placement, since control of the devices is no longer dependent on power wires and power outlets, and allowing greater device portability, at least for those devices which may be battery-powered such as switches and sensors. Typical examples for such wireless personal networks (WPAN) are ZigBee (IEEE 802.15.4), Bluetooth, HomeRF or Wi-Fi networks.
In many cases, the transmit range of the air interfaces between the devices is smaller than the dimensions of the network, and it may be necessary, for an application-level connection, for a first device to establish a network path (in the following also referred to as “route”) to a second device via a number of third devices which pass a message from the first device to the second device. Such a network in which, on application-level, a sending device transmits a message to a receiving device using other devices as intermediate stations is referred to as a “multi-hop” network. Thereby, the network path between a sending and a receiving device may be established in a self-organizing manner according to rules of the relevant network standard. For it, physical connections—i.e. the fact of devices being directly in each others radio range, especially in mesh networks—and logical connections—i.e. the special relationships between the nodes, especially in a tree-based topology—are used. The physical and logical connections between the devices may be established in an initialisation process, for example, when a device joins the network. In another example, a network path may be established when it is needed, in an ad-hoc manner, in particular when a previous network path breaks down, for example, owing to the failure of an intermediate device in the path.
Currently, ZigBee is the only standard low-power, low-footprint WPAN technology that allows self-organization to a large extent. ZigBee offers two ways of establishing the network topology: ‘DirectJoin’ and free association, whereby in both cases a tree structure may be established, which is the preferred allocation for smaller networks, or a stochastic structure may be established which can be a favourable solution for larger networks. The device at which the new device joins is called the ‘parent device’ of the joined device (‘child device’).
DirectJoin method requires the user to pre-define the parent-child relationships for all devices in the network. Besides placing a high burden on the user (especially in large networks with thousands of nodes), this requires expert tools to measure radio range and/or expert knowledge on wireless propagation, and is associated with a high maintenance effort when alterations or adaptations must be made to the network. This is undesirable in the instable, multipath-susceptible wireless environment.
The free association procedure allows a seeking device (either a completely new one or one that had been in the network before) intending to join the network, to select a parent node according to pre-defined criteria. In the ZigBee standard, both reduced-function ZigBee End Devices (also referred to as ‘ZED’ in the following) and ZigBee Routers (also referred to as ‘ZR’ in the following) attempt to select as a parent a router for which all the following conditions are true:
1. The router belongs to the network identified by a particular network identifier (‘ExtendedPANId parameter’ or ‘EPID’ in the ZigBee-standard);
2. The router is open to join requests and advertises capacity of the correct device type (ZigBee Router or ZigBee End Device);
3. The announced ‘update id’ is recent;
4. The link quality for frames received from this device is such that a link cost of at most 3 is produced. [ZigBee-2007, 053474r17, sec. 3.6.1.4.1.1, p. 352, 1. 1 ff],
whereby, the link cost is calculated as
                              C          ⁢                      {            1            }                          =                  {                                                                      7                  ,                                                                                                      min                  ⁡                                      (                                          7                      ,                                              round                        ⁡                                                  (                                                      1                                                          p                              1                              4                                                                                )                                                                                      )                                                                                                          (        1        )            
where p1 is the probability of packet delivery on the link 1 and can be arrived at by analyzing packet error rates and/or signal properties such as Energy Detection and/or Signal-to-Noise ratio values; at ZigBee implementer's discretion.
5. If more than one device meets these four requirements, then, according to the ZigBee standard, the joining device shall select the parent with the smallest tree depth. [ZigBee-2007, 053474r17, sec. 3.6.1.4.1.1, p. 352, 1.18 ff]. If more than one potential parent has the smallest depth the device is free to choose from these.
On initial network formation, the devices will not yet have a good estimate of the link cost since very few packets will have been sent/received by then. When the default value of 7 is taken for the link cost (see equation (1)), all possible parents meet the requirements 1-4 with a tie on the fourth requirement, which will effectively cause the node to apply the fifth rule only, i.e. to choose the parent highest in the network tree (with the lowest tree depth).
Therefore, the above free association method may result in network topologies that are very dense in close proximity to the tree root—i.e. to the ZigBee PAN Coordinator (in the following also named ‘ZC’), which forms the root of the network logical structure (esp. tree)—and may result in parent-child links of potentially poor quality (at least, higher quality links might be available). As a result, the performance of the network may be less than optimal with regard to both delay and reliability of packet delivery. This is especially true for the networks incorporating ZEDs, which are compelled by the standard to communicate solely via their parent.
Many ZED-like reduced-function, battery-operated devices are expected in the ZigBee-like networks, because the strength of the wireless control lies in its independence of mains power. The control devices—switches, sensors, remote controllers (RC), etc., using radio mostly in reaction to user input or sensed environmental change—are expected to be battery-operated ZEDs. Multiple devices may be controlled by them, including packet-loss sensitive and time-critical applications like light switching, requiring—despite ZED's reduced functionality—certain network performance level. Furthermore, it is desirable to maximize network reliability, and thereby conserve ZED's energy, by keeping the number of re-transmissions associated with a command to a minimum.
Therefore, it is an object of the invention to provide a more intelligent self-organization method for the formation of wireless multi-hop networks, in particular ZigBee-type networks that include ZEDs, and to provide a device comprising a network interface for connecting a wireless multi-hop network according to this method.